Bug 1047521 - Set up a way for people to create webmaker accounts from welcome.wmo

<https://bugzilla.mozilla.org/show_bug.cgi?id=1047521>
I'm bouncing this question to @jbuck ...

I believe the audience should always match what's in the URL bar, but I'm not an expert on persona.

Also, have we got this covered for `window.location.origin`? (https://gist.github.com/hbogs/7908703)